.TH std::jthread::jthread 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::jthread::jthread \- std::jthread::jthread

.SH Synopsis
   jthread() noexcept;                        \fB(1)\fP \fI(since C++20)\fP
   jthread( jthread&& other ) noexcept;       \fB(2)\fP \fI(since C++20)\fP
   template< class F, class... Args >         \fB(3)\fP \fI(since C++20)\fP
   explicit jthread( F&& f, Args&&... args );
   jthread( const jthread& ) = delete;        \fB(4)\fP \fI(since C++20)\fP

   Constructs new std::jthread object.

   1) Creates new std::jthread object which does not represent a thread.
   2) Move constructor. Constructs the std::jthread object to represent the thread of
   execution that was represented by other. After this call other no longer represents
   a thread of execution.
   3) Creates new std::jthread object and associates it with a thread of execution.

   The new thread of execution starts executing:

   std::invoke(decay-copy(std::forward<F>(f)), get_stop_token(), (until C++23)
               decay-copy(std::forward<Args>(args))...)
   std::invoke(auto(std::forward<F>(f)), get_stop_token(),       (since C++23)
               auto(std::forward<Args>(args))...)

   if the expression above is well-formed, otherwise starts executing:

   std::invoke(decay-copy(std::forward<F>(f)),           (until C++23)
               decay-copy(std::forward<Args>(args))...).
   std::invoke(auto(std::forward<F>(f)),                 (since C++23)
               auto(std::forward<Args>(args))...).

   The calls of decay-copy are evaluated
   (until C++23)
   The values produced by auto are materialized
   (since C++23) in the current thread, so that any exceptions thrown during evaluation
   and copying/moving of the arguments are thrown in the current thread, without
   starting the new thread.
   These overloads participate in overload resolution only if std::remove_cvref_t<F> is
   not the same type as std::jthread.
   If any of the following is false, the program is ill-formed:
     * std::is_constructible_v<std::decay_t<F>, F>
     * (std::is_constructible_v<std::decay_t<Args>, Args> && ...)
     * std::is_invocable_v<std::decay_t<F>, std::decay_t<Args>...> ||
       std::is_invocable_v<std::decay_t<F>, std::stop_token, std::decay_t<Args>...>
   The completion of the invocation of the constructor synchronizes with the beginning
   of the invocation of the copy of f on the new thread of execution.
   4) The copy constructor is deleted; threads are not copyable. No two std::jthread
   objects may represent the same thread of execution.

.SH Parameters

   other - another std::jthread object to construct this std::jthread object with
   f     - Callable object to execute in the new thread
   args  - arguments to pass to the new function

.SH Postconditions

   1) get_id() equal to std::jthread::id() (i.e. joinable() returns false) and
   get_stop_source().stop_possible() is false.
   2) other.get_id() equal to std::jthread::id() and get_id() returns the value of
   other.get_id() prior to the start of construction.
   3) get_id() not equal to std::jthread::id() (i.e. joinable() returns true), and
   get_stop_source().stop_possible() is true.

.SH Exceptions

   3) std::system_error if the thread could not be started. The exception may represent
   the error condition std::errc::resource_unavailable_try_again or another
   implementation-specific error condition.

.SH Notes

   The arguments to the thread function are moved or copied by value. If a reference
   argument needs to be passed to the thread function, it has to be wrapped (e.g. with
   std::ref or std::cref).

   Any return value from the function is ignored. If the function throws an exception,
   std::terminate is called. In order to pass return values or exceptions back to the
   calling thread, std::promise or std::async may be used.

.SH Example


// Run this code

 #include <chrono>
 #include <iostream>
 #include <thread>
 #include <utility>

 using namespace std::literals;

 void f1(int n)
 {
     for (int i = 0; i < 5; ++i)
     {
         std::cout << "Thread 1 executing\\n";
         ++n;
         std::this_thread::sleep_for(10ms);
     }
 }

 void f2(int& n)
 {
     for (int i = 0; i < 5; ++i)
     {
         std::cout << "Thread 2 executing\\n";
         ++n;
         std::this_thread::sleep_for(10ms);
     }
 }

 class foo
 {
 public:
     void bar()
     {
         for (int i = 0; i < 5; ++i)
         {
             std::cout << "Thread 3 executing\\n";
             ++n;
             std::this_thread::sleep_for(10ms);
         }
     }
     int n = 0;
 };

 class baz
 {
 public:
     void operator()()
     {
         for (int i = 0; i < 5; ++i)
         {
             std::cout << "Thread 4 executing\\n";
             ++n;
             std::this_thread::sleep_for(10ms);
         }
     }
     int n = 0;
 };

 int main()
 {
     int n = 0;
     foo f;
     baz b;
     std::jthread t0; // t0 is not a thread
     std::jthread t1(f1, n + 1); // pass by value
     std::jthread t2a(f2, std::ref(n)); // pass by reference
     std::jthread t2b(std::move(t2a)); // t2b is now running f2(). t2a is no longer a thread
     std::jthread t3(&foo::bar, &f); // t3 runs foo::bar() on object f
     std::jthread t4(b); // t4 runs baz::operator() on a copy of object b
     t1.join();
     t2b.join();
     t3.join();
     std::cout << "Final value of n is " << n << '\\n';
     std::cout << "Final value of f.n (foo::n) is " << f.n << '\\n';
     std::cout << "Final value of b.n (baz::n) is " << b.n << '\\n';
     // t4 joins on destruction
 }

.SH Possible output:

 Thread 2 executing
 Thread 1 executing
 Thread 4 executing
 Thread 3 executing
 Thread 3 executing
 Thread 4 executing
 Thread 2 executing
 Thread 1 executing
 Thread 3 executing
 Thread 1 executing
 Thread 4 executing
 Thread 2 executing
 Thread 3 executing
 Thread 1 executing
 Thread 4 executing
 Thread 2 executing
 Thread 3 executing
 Thread 1 executing
 Thread 4 executing
 Thread 2 executing
 Final value of n is 5
 Final value of f.n (foo::n) is 5
 Final value of b.n (baz::n) is 0

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to              Behavior as published              Correct behavior
                       overload \fB(3)\fP directly required (the decayed
   LWG 3476 C++20      types of)                                       removed these
                       F and the argument types to be move             requirements^[1]
                       constructible

    1. ↑ The move-constructibility is already indirectly required by
       std::is_constructible_v.

.SH See also

   constructor   constructs new thread object
                 \fI(public member function of std::thread)\fP
   C documentation for
   thrd_create
